In order to attenuate in particular impact forces in vehicles use is made of springy arrangements such as for instance helical springs and leafsprings. Helical springs are most commonly made of round material and the resilient properties of the steel from which they are made are achieved above all by cold working such as drawing of the material. For vehicles the wheels of which are subject to low loads these helical springs are well suited. As regards heavier vehicles use is made of springs having a largercross-sectional area such as leafsprings of various designs. Apart from many other advantages it is a characteristic of the leafsprings that their stability in other directions of motion than the one in which the spring is intended to act is high.
Since the moment of forces acting on leafsprings is greatest at their centre, that is where one provides the largest cross-sectional area. This can be done in different ways. One can make the spring leaf widest at the centre and tapering towards the ends. This spring design is space-consuming and entails great losses of material in the process of manufacture. The spring leaf can be made thicker at the centre. Such springs are often referred to as parabolic leafsprings and have recently been gaining in importance. Parabolic leafsprings require relatively little space but on the other hand their production is costly. The most common type of leafsprings is the so-called laminated leafspring and above all the type produced from flat bars, which are cut to different lengths and stacked on top of one another. The top leaf is longest and has fastening eyes at the ends. The length of the leaf is then reduced towards the centre, and the leaf package is held together on the one hand by means of a pin passing through the centre and on the other hand by a number of yokes placed about the package between the centre and the fastening eyes. These springs are cheapest from the point of view of production. Owing to the mutual friction between the leaves the spring action is also subject to hysteresis, which causes the motion of the spring to be attenuated.
A good product for the production of springs has to satisfy stringent requirements of repeatability in order to avoid a lack of symmetry in the spring action. Apart from small dimensional variation also the variation in the characteristics of the steel should be small.
The most important characteristics of steel used for springs are a high elastic limit and a high fatigue limit.
The energy storing capacity of the spring material is proportional to the square of the elastic limit (R.sub.E.sup.2). By elastic limit is meant the maximum specific loading to which the material can be exposed for its return to the initial position without deformation of the material. With many types of steel the values of the elastic limit and the yield stress are almost identical.
The fatigue limit follows in priciple the elastic limit but, in addition, depends above all on the structural homogeneity and the surface finish, which benefits from a smooth surface free of decarbonisation layers and defects such as slag inclusions etc.
Especially with relatively coarse types of steel use is generally made of hardening and tempering in order to increase, inter alia, the elastic limit. In this connection a full hardening steel is indicated, i.e. a steel with a composition so adjusted to the thickness of the material that quenching results in the production of martensite also at the centre of the section. After hardening the material is tempered at temperatures generally in the region of 400.degree.-500.degree. C. in order to achieve a certain degree of toughness. Unfortunately, however, such tempering causes the elastic limit to drop.
So far the most important means with a view to achieving a high elastic limit has consisted in producing spring steel with relatively high carbon contents. Also the hardenability increases with the carbon content.
Among the types of steel currently used for springs mention may be made of SS 2090 and SS 2230. The designations are taken from the Swedish Standard.
SS 2090 contains 0.52-0.60% C. and 1.5-2.0% Si. As a result of the high Si content the full hardening capacity is increased. However, Si used by way of alloying constituent is relatively expensive.
SS 2230 contains 0.48-0.55% C., slightly more silicium as well as 0.70-1.00% Mn, 0.90-1.20% Cr and 0.10-0.20% V. This steel is used for relatively large springs. The increased contents of silicium, manganese and chromium contribute above all to the hardenability whereas vanadium is added for grain refinement.
Owing to the alloying additives the steel is costly to produce. In addition, it is relatively difficult to work by cutting and shearing drilling or stamping operations such as the drilling of holes through the leaves in order to hold together the spring package. With both of the above types of steel there is also a danger of surface decarbonisation, reducing inter alia the fatigue limit.